kselftest/arm64: Test FPSIMD format data writes via NT_ARM_SVE in fp-ptrace

kselftest/arm64: Test FPSIMD format data writes via NT_ARM_SVE in fp-ptrace

The NT_ARM_SVE register set supports two data formats, the native SVE one
and an alternative format where we embed a copy of user_fpsimd_data as used
for NT_PRFPREG in the SVE register set. The register data is set as for a
write to NT_PRFPREG and changes in vector length and streaming mode are
handled as for any NT_ARM_SVE write. This has not previously been tested by
fp-ptrace, add coverage of it.

We do not support writes in FPSIMD format for NT_ARM_SSVE so we skip the
test for anything that would leave us in streaming mode.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250718-arm64-fp-ptrace-sve-fpsimd-v1-1-7ecda32aa297@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>